Preparation And Electrical Properties Of （Bi,Gd）FeO₃ Gradient Films

Lead-free ferroelectric with distorted perovskite structure at room temperature,Bismuth ferrite（BiFeO₃,BFO）has higher Curie temperature（～830℃）and larger theoretical polarization intensity（～100μC/cm²）than other ferroelectric materials,which is expected to be widely used in sensors,actuators and other fields.BFO films have excellent electrical properties,and the advantages of small size,easy preparation of small devices and can be matched with integrated semiconductor processes.BFO films are more suitable for the future development trend of miniaturization of electronic devices,and gradually become a hot material in the field of micro-electromechanical systems（MEMS）.The composition gradient thin film based on interlayer strain is an effective means to improve the ferroelectric and piezoelectric properties of materials.By designing different elements inside the film,the microstructure of the film can be adjusted to a certain extent and the electrical properties of the film can be affected.In this paper,the microstructure and electrical properties of（Bi,Gd）FeO₃-based thin films are systematically studied by designing the gradient distribution of Gd content inside the films and constructing the compositional gradient films based on interlayer strain.The results obtained are as follows:（1）Single homogeneous Bi_0.97Gd_0.03FeO₃（BGFO-03）,Bi_0.91Gd_0.09FeO₃（BGFO-09）,Bi_0.85Gd_0.15FeO₃（BGFO-15）thin films,as well as the gradient BGFO-up(Pt/Bi_0.97Gd_0.03FeO₃/Bi_0.91Gd_0.09FeO₃/Bi_0.85Gd_0.15FeO₃)film and the gradient BGFO-down(Pt/Bi_0.85Gd_0.15FeO₃/Bi_0.91Gd_0.09FeO₃/Bi_0.97Gd_0.03FeO₃)film are prepared on Pt/Ti/Si O₂/Si substrate by a sol-gel method.All the prepared films show rhombohedral R3c structure,and no impurity is generated.The BGFO-down film exhibits good self-polarization characteristics.Under the electric field of 875 k V/cm,the internal bias field E_int is as high as 206 k V/cm.The electrical performance test results show that the piezoelectric coefficient d₃₃ is high up to 174pm/V,and the film exhibits good insulation performance(J～1.12×10^-6 A/cm²@200 k V/cm).The degree of self-polarization of the films was characterized by testing the energy exchange characteristics of the films.BGFO-down film has the best piezoelectric output performance,with an output voltage of 4.8V at a bending Angle of 60°,which is 1.7,1.9 and 2.5 times that of homogeneous BGFO-03,BGFO-09 and BGFO-15 films,respectively.（2）Conduct in-depth study on gradient BGFO-down thin films,and regulate the microstructures and electrical properties of BGFO-down thin films by changing film thickness,controlling annealing temperature and selecting different substrate materials.The results show that the film with a thickness of 800 nm has the best overall performance(P_r～68.5μC/cm²,E_int～106 k V/cm@1600 k V/cm).The effect of different annealing temperature on the film is studied.BGFO-down film deposited at 500℃has the largest E_int and the smallest leakage.Compared with FTO and ITO substrates,BGFO-down film deposited on Pt/Ti/Si O₂/Si substrates has the largest E_int.The working stability of BGFO-down thin film was investigated.The experimental results show that the P-E curves of the sample remain unchanged in the frequency range of 1 k Hz to 50 k Hz and the temperature range of 25℃to 200℃.The self-polarization performance of the film samples does not deteriorate significantly after 10⁹ fatigue cycles or 180 days of placement,indicating that it still has a stable self-polarization state in the actual working situation.Finally,large-scale BGFO-down thin films were prepared on 6-inch wafers by a sol-gel method.The variation range of the internal bias field E_int was less than 5%,showing good self-polarization uniformity.